The Moon may hold much more water than we think
Scientists
have long suspected the Moon holds sizable reserves of water, secreted as ice
in the deep cold of permanently shadowed craters near the poles. Two new
studies tell us more about the possible extent of those reserves. One suggests
the shadowy polar caches may cover an area equivalent to the states of
Connecticut and Massachusetts combined; the other reveals traces of water
elsewhere on the Moon’s surface, trapped in rocks or between the grains of
lunar soil. That’s welcome news for NASA, which plans to return astronauts to
the lunar surface in 2024 as a first step toward a permanent outpost and
eventual journeys to Mars.
Water on
the Moon would be good for more than just drinking. It can be chemically split
into hydrogen and oxygen, yielding components for rocket fuel—and breathable
air. Having ready supplies of water on the lunar surface would be a boon for
colonists there, because it is so expensive to transport from Earth. A 2008–09
orbital expedition detected
the signature of water in shadowy lunar hollows. But how much is
there?
To find
out, Paul Hayne, a planetary scientist at the University of Colorado, Boulder,
and his colleagues estimated the number and size of permanently shadowed polar
regions, where temperatures remain below –163°C. Any water in those areas would
have likely come from meteorites, comets, and other objects that once slammed
into the Moon’s surface. Most water would have vaporized, but some would have
drifted through the sparse atmosphere until it reached a shadowy nook, only to
deposit as frost on the ever-frigid rocks and soil.
Analyzing
high-resolution lunar images, the team calculated that the Moon’s polar regions
host about 40,000 square kilometers of permanently shadowed areas that could contain
water, from kilometers-wide craters to shallow depressions in the
meteorite-gouged terrain, they report today in Nature Astronomy.
About 60% of that area is in the Moon’s southern hemisphere.
Although
the researchers did not estimate how much water might be present, anything in
these regions should be easy to harvest, Hayne says. It might be as simple as
having a lunar rover drag icy rocks and soil into a sunlit spot and collect
water as it evaporates. Just last week, NASA
announced a $47 million commercial contract to send an ice-seeking
drill to the Moon in 2023.
A second
study follows tantalizing evidence that there may be water elsewhere on the
Moon. As early as 2009, scientists detected a spectral signature suggesting the
presence of water in sunlit areas of the Moon. But because that signature—a particular
wavelength of infrared radiation—can also be absorbed by substances other than
water, it wasn’t indisputable evidence.
So Casey
Honniball, a lunar scientist at NASA’s Goddard Space Flight Center, and
colleagues sought a different spectral signature that could be generated only
by water. They used NASA’s Stratospheric Observatory for Infrared Astronomy
telescope, an infrared instrument mounted in a converted Boeing 747 that
cruises at about 14,000 meters, to look for infrared light at a wavelength emitted
by water molecules. Their flights found that each kilogram of lunar soil along
two narrow swaths of the Moon’s surface contains between 100 milligrams and 400
milligrams of water, or about one
raindrop’s worth, the team reports today in Nature Astronomy.
Almost all of that water, they note, would be locked in shadowed areas between
grains of lunar soil, or trapped in glassy materials created when
micrometeorites smacked the lunar surface.
The
team’s finding is “very exciting,” Hayne says. He adds that if the water is so
trapped, it would be relatively easy to melt the glassy materials and, in
essence, “mine” the water.
That
mining would be a boon to future Moon missions, says Jacob Bleacher, chief
exploration scientist for the Human Exploration and Operations Mission
Directorate at NASA headquarters. Understanding where water is will not only
help NASA decide where to send astronauts, but it could also lighten their
payloads—and make more room for scientific equipment to be carried aloft.
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